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Chapter 9: Cellular Respiration: Harvesting Chemical Energy Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Hans Adolf Krebs – received the Nobel Prize in 1953 for his work on the series of chemical reactions known as the tricarboxylic acid cycle (also called the citric acid cycle, or Krebs cycle). This is the basic system for the essential pathway of oxidation within the cell. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Figure 9.1 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Figure 9.1 The giant panda is consuming fuel to power the work of life Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Figure 9.2 Light energy ECOSYSTEM Photosynthesis in chloroplasts CO2 H2O Cellular respiration in mitochondria ATP Heat energy Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Organic O2 molecules ATP powers most cellular work • Energy flows into an ecosystem as sunlight and leaves as heat • Photosynthesis generates O2 and organic molecules, which are used in cellular respiration • Cells use chemical energy stored in organic molecules to regenerate ATP, which powers work Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings © 2011 Pearson Education, Inc. Catabolic Pathways and Production of ATP • Fermentation is a partial degradation of sugars that occurs without O2 • Aerobic respiration consumes organic molecules and O2 and yields ATP Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings © 2011 Pearson Education, Inc. Harvesting of energy from glucose via cellular respiration has three stages: – Glycolysis (breaks down glucose into two molecules of pyruvate) – The citric acid cycle (completes the breakdown of glucose) – Oxidative phosphorylation (accounts for most of the ATP synthesis) Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings © 2011 Pearson Education, Inc. Figure 9.UN05 1. Glycolysis (color-coded teal throughout the chapter) 2.Pyruvate oxidation and the citric acid cycle (color-coded salmon) 3. Oxidative phosphorylation: electron transport and chemiosmosis (color-coded violet) Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Figure 9.6-1 Electrons carried via NADH Glycolysis Glucose Pyruvate CYTOSOL MITOCHONDRION ATP Substrate-level phosphorylation Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Figure 9.6-2 Electrons carried via NADH and FADH2 Electrons carried via NADH Glycolysis Glucose Pyruvate CYTOSOL Pyruvate oxidation Acetyl CoA Citric acid cycle MITOCHONDRION ATP ATP Substrate-level phosphorylation Substrate-level phosphorylation Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Figure 9.6-3 Electrons carried via NADH and FADH2 Electrons carried via NADH Glycolysis Glucose Pyruvate CYTOSOL Pyruvate oxidation Acetyl CoA Citric acid cycle Oxidative phosphorylation: electron transport and chemiosmosis MITOCHONDRION ATP ATP ATP Substrate-level phosphorylation Substrate-level phosphorylation Oxidative phosphorylation Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Oxidative phosphorylation accounts for almost 90% of the ATP generated by cellular respiration • A smaller amount of ATP is formed in glycolysis and the citric acid cycle by substrate-level phosphorylation (~10%) • For each molecule of glucose degraded to CO2 and water by respiration, the cell makes obtains ~ 32 molecules of ATP Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings © 2011 Pearson Education, Inc. Figure 9.8 Energy Investment Phase Glucose 2 ADP 2 P 2 ATP used Energy Payoff Phase 4 ADP 4 P 2 NAD+ 4 e 4 H+ 4 ATP formed 2 NADH 2 H+ 2 Pyruvate 2 H2O Net Glucose 4 ATP formed 2 ATP used 2 NAD+ 4 e 4 H+ Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 2 Pyruvate 2 H2O 2 ATP 2 NADH 2 H+ Oxidation of Pyruvate to Acetyl CoA • Before the citric acid cycle can begin, pyruvate must be converted to acetyl Coenzyme A (acetyl CoA), which links glycolysis to the citric acid cycle Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings © 2011 Pearson Education, Inc. The Citric Acid Cycle • The citric acid cycle, also called the Krebs cycle, completes the break down of pyruvate to CO2 • The cycle oxidizes organic fuel derived from pyruvate, generating 1 ATP, 3 NADH, and 1 FADH2 per cycle Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings © 2011 Pearson Education, Inc. Figure 9.11 Pyruvate CO2 NAD CoA NADH + H Acetyl CoA CoA CoA Citric acid cycle 2 CO2 3 NAD FADH2 3 NADH FAD + 3 H ADP + P i ATP Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Concept 9.4: During oxidative phosphorylation, chemiosmosis couples electron transport to ATP synthesis • Following glycolysis and the citric acid cycle, NADH and FADH2 account for most of the energy extracted from food • These two electron carriers donate electrons to the electron transport chain, which powers ATP synthesis via oxidative phosphorylation Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings © 2011 Pearson Education, Inc. Chemiosmosis: The Energy-Coupling Mechanism • Electron transfer in the electron transport chain causes proteins to pump H+ from the mitochondrial matrix to the intermembrane space • H+ then moves back across the membrane, passing through the proton, ATP synthase • ATP synthase uses the exergonic flow of H+ to drive phosphorylation of ATP • This is an example of chemiosmosis, the use of energy in a H+ gradient to drive cellular work Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings © 2011 Pearson Education, Inc. INTERMEMBRANE SPACE Figure 9.14 H Stator Rotor This diagram shows how a protein motor is used as a rotor which uses the hydrogen ion energy to generate energy to convert ADP into ATP Internal rod Catalytic knob ADP + Pi The stator is the stationary part of a rotor system, found in biological rotors ATP MITOCHONDRIAL MATRIX Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings An Accounting of ATP Production by Cellular Respiration • During cellular respiration, most energy flows in this sequence: glucose NADH electron transport chain proton-motive force ATP • About 34% of the energy in a glucose molecule is transferred to ATP during cellular respiration, making ~ 32 ATP Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings © 2011 Pearson Education, Inc. Figure 9.16 Electron shuttles span membrane 2 NADH Glycolysis 2 Pyruvate Glucose MITOCHONDRION 2 NADH or 2 FADH2 2 NADH Pyruvate oxidation 2 Acetyl CoA 6 NADH Citric acid cycle 2 ATP 2 ATP Maximum per glucose: CYTOSOL Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings About 30 or 32 ATP 2 FADH2 Oxidative phosphorylation: electron transport and chemiosmosis about 26 or 28 ATP ATP yield per molecule of glucose at each stage of cellular respiration can vary Electron shuttles span membrane CYTOSOL MITOCHONDRION 2 NADH or 2 FADH2 2 NADH 2 NADH Glycolysis Glucose 2 Pyruvate 6 NADH Citric acid cycle 2 Acetyl CoA + 2 ATP by substrate-level phosphorylation Maximum per glucose: 2 FADH2 Oxidative phosphorylation: electron transport and chemiosmosis + 2 ATP + about 32 or 34 ATP by oxidative phosphorylation, depending by substrate-level on which shuttle transports electrons phosphorylation from NADH in cytosol About 36 or 38 ATP Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Concept 9.5: Fermentation and anaerobic respiration enable cells to produce ATP without the use of oxygen • Without O2, the electron transport chain will cease to operate • In that case, glycolysis couples with fermentation or anaerobic respiration to produce ATP • Two common types are alcohol fermentation and lactic acid fermentation Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings © 2011 Pearson Education, Inc. Figure 9.17 2 ADP 2 P Glucose 2 ADP 2 P 2 ATP i Glycolysis Glucose 2 ATP i Glycolysis 2 Pyruvate 2 NAD 2 Ethanol 2 NADH 2 H 2 NAD 2 CO2 2 Acetaldehyde (a) Alcohol fermentation Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 2 NADH 2 H 2 Pyruvate 2 Lactate (b) Lactic acid fermentation • In alcohol fermentation, pyruvate is converted to ethanol in two steps, with the first releasing CO2 • Alcohol fermentation by yeast is used in brewing, winemaking, and baking Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings © 2011 Pearson Education, Inc. • In lactic acid fermentation, pyruvate is reduced to NADH, forming lactate as an end product, with no release of CO2 • Lactic acid fermentation by some fungi and bacteria is used to make cheese and yogurt • Human muscle cells use lactic acid fermentation to generate ATP when O2 is scarce Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings © 2011 Pearson Education, Inc. Comparing Fermentation with Anaerobic and Aerobic Respiration • All use glycolysis (net ATP =2) to oxidize glucose and harvest chemical energy of food • Cellular respiration produces 32 ATP per glucose molecule; fermentation produces 2 ATP per glucose molecule Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings © 2011 Pearson Education, Inc. • Obligate anaerobes carry out fermentation or anaerobic respiration and cannot survive in the presence of O2 • Yeast and many bacteria are facultative anaerobes, meaning that they can survive using either fermentation or cellular respiration • In a facultative anaerobe, pyruvate is a fork in the metabolic road that leads to two alternative catabolic routes Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings © 2011 Pearson Education, Inc. Figure 9.18 Glucose Glycolysis CYTOSOL Pyruvate No O2 present: Fermentation O2 present: Aerobic cellular respiration MITOCHONDRION Ethanol, lactate, or other products Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Acetyl CoA Citric acid cycle Figure 9.19 The catabolism of various molecules from food Proteins Carbohydrates Amino acids Sugars Glycolysis Glucose Glyceraldehyde-3- P NH3 Pyruvate Acetyl CoA Citric acid cycle Oxidative phosphorylation Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Fats Glycerol Fatty acids